@Di1981 

Hi,thanks for posting question here.

As we do not know the model number of your DSL modem router, it is hard to provide detailed instructions.

Below are the common instructions for your reference:

TP-Link DSL modem routers support in 2 working modes, DSL modem router and wireless router.
If you don't have an NBN box at home and you want to connect a DSL cable (phone cable) to the DSL modem router, configure the modem router as DSL modem router mode.
If you already have an NBN box and want to connect an Ethernet cable to the DSL modem router, set it up as a regular wireless router mode.
Here is a FAQ which provides a detailed setup process: via Web UI or Tether APP
Or you could also refer to the Videos: DSL modem router mode and Wireless router mode.

Note, please take factory reset on your DSL modem router first before reconfiguration.

And, it is also suggested to contact Support Hotline directly for direct help.